Step 1. The very first step is to view the current edition. So open Command Prompt or Windows PowerShell in elevated mode (Run as Administrator) and type, DISM /online /Get-CurrentEditon as shown below. As you can see the current edition is Datacenter Evaluation.

Step 2. You can also view to which target edition you can upgrade. To view the possible upgrade path, type DISM /online /Get-TargetEditions on elevated command prompt as shown below. As you can see the possible upgrade option is, Datacenter edition.

Step 3. To upgrade type DISM /online /Set-Edition:ServerDatacenter /productkey:XXXX-XXXX-XXXX-XXXX-XXXX /AcceptEula. X is your product key. As you can see below, it this command will upgrade the edition of Server to full version. Press Y to reboot.